The present invention relates to a three-dimensional cam having a surface that varies continuously in the axial direction. More particularly, the present invention relates to a three-dimensional engine valve cam having a profile for controlling the opening and closing of engine valves in accordance with the operating state of the engine. The present invention also pertains to a method for measuring three-dimensional cams, measuring tools for testing profiles of three-dimensional cams, and an apparatus for measuring three-dimensional cams. The present invention also relates to an engine valve drive apparatus employing such three-dimensional cams.
FIG. 24 shows a prior art valve drive apparatus that continuously varies the opening and closing timing and lift amount of engine intake valves and engine exhaust valves. Japanese Examined Patent Publication No. 7-45803 and Japanese Unexamined Patent Publication No. 9-32519 describes such apparatus. As shown in FIG. 24, two valves 543, which are either intake valves or exhaust valves, are provided for a single cylinder of an engine. Each valve 543 is connected to and driven by a three-dimensional cam 540, which is fixed to a camshaft 542. The cam 540 has a cam surface 540a used to drive the valves 543. A cam nose, the radius of which changes continuously in the direction of the camshaft axis Y of the camshaft 542, is defined on the cam surface 540a. The shifting mechanism 541 shifts the camshaft 542 to displace each cam 540 within a range denoted by D. As the cam 540 shifts, the nose radius of the cam surface 540a changes continuously. This varies the lift amount and opening and closing timing of the associated valve 543. The change in the lift amount (lift control amount) occurs within a range defined between the maximum and minimum values of the cam nose radius. The shifting of the camshaft 542 along the axis Y is controlled so that the maximum lift amount of each valve 543 is small when the engine is in a low speed range and is large when the engine is in a high speed range. This improves engine performance, especially in terms of torque and stability.
As shown in FIG. 24, a valve lifter 549 is arranged between each valve 543 and the associated three-dimensional cam 540. A cam follower seat 544 is defined in the top center surface of each valve lifter 549. A cam follower 545 is pivotally received in each follower seat 544 so that the valve lifter 549 can follow the cam surface 540a of the associated cam 540.
Each cam follower 545 has a flat slide surface 545a, which slides along the associated cam surface 540. The shape of the cam follower 545 is shown enlarged in FIGS. 25(a) and 25(b). As shown in FIG. 25(a), the cam follower 545 has a semicircular cross-section. FIG. 25(b) is a side view of the cam follower 545.
As shown in FIG. 26, the cam follower 545 has a first edge 545b and a second edge 545c that engage the cam surface 540a. Contact between the cam follower 545 and the cam surface 540a occurs between the first edge 545b and the second edge 545c. The first edge 545b contacts the cam surface 540a where the cam nose radius is smaller than that where the second edge 545c contacts the cam surface 540a.
FIG. 27 is a perspective view showing the cam surface 540a. The uniformly dashed line represents one axial end of the cam 540, or cam profile 547, where the cam nose radius is smallest. The long and short dashed line represents the other axial end of the cam 540, or cam profile 548, where the cam nose radius is greatest. As apparent from the drawing, the profile of the cam 540 varies continuously in the axial direction. Each elemental line 546 shown in the drawing represents the same angular position on the cam surface 540a. In other words, the lines 546 represent intersections between the cam surface and planes that include the axis Y. Although the drawing shows a limited number of lines 546, an infinite number of lines 546 may be defined along the cam surface 540. Hence, the cam follower 545 comes into linear contact with the cam surface 540a along part of each line 540.
As shown in FIG. 26, when the three-dimensional cam 540 shifts along the axis Y, the slide surface 545a between the first and second edges 545a, 545b of the cam follower 545 is in linear contact with and moves relative to the cam surface 540a. Lubricating oil is removed from the cam surface 540a when relative movement takes place between the cam follower 545 and the cam surface 540a. This occurs especially when the second edge 545c scrapes off the lubricating oil from the cam surface 540a as the cam follower 545 shifts along the cam surface 540a from the smaller radius side to the larger radius side. As a result, lubrication between the second edge 545c and the cam surface 540a becomes insufficient. This may lead to wear of the second edge 545c and the cam surface 540a.
Generally, the small radius side of the cam 540 is used more frequently than the large radius side. Therefore, a difference in wear occurs along the cam surface 540a in the axial direction Y. The wear difference causes the cam surface 540a to become uneven. An uneven cam surface 540a may interfere with the movement of the second edge 545c and thus hinder with smooth shifting of the opening and closing timing and lift amount of the associated valve 543.
Additionally, the cam surface 540 is machined with precision so that the surface 540a is straight as shown in FIG. 27. However, tolerances permitted during machining of the cam surface 546 may result in a slight concavity in surface 540a, as shown in FIG. 28. In such case, only the first and second edges 545b, 545c of the cam follower 545 contact the cam surface 540a. This may cause the first and second edges 545b, 545c to scratch the cam surface 540a during rotation of the cam 545 or cause biased wear of the cam follower 545 at the edges 545b, 545c.
When scratches are formed in the cam surface 540a, the scratches may interfere with axial movement of the three-dimensional cam 540. This would hinder with smooth varying of the opening and closing timing and lift amount of the associated valve 543.